[username190]

> I think only one of the 2 slots fully supports LTE.

Both of the slots support LTE; OnePlus has made some frustrating power saving decisions though, which force the secondary (non-active) SIM to 3G when it's not in use.

This can be disabled (at least on my 7 Pro running Android 10) via logkit following these steps[0]. Once disabled, the phone will fully support VoLTE on both SIMs.

[0] https://forums.oneplus.com/threads/dual-4g-networks-setting....